American Scientist

(Excerpted from The Sound Book: The Science of the Sonic Wonders of the World by Trevor Cox. Copyright 2014 by Trevor Cox. With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved. This selection may not be reproduced, stored in a retrieval system, or transmitted in any form by any means without prior written permission of the publisher.)

While I was on an expedition to record singing sand dunes, I experienced something quite rare: complete silence. The scorching summer heat kept visitors away. Most of the time my recording companion, Diane Hope, and I were on our own. We camped at the foot of Kelso Dunes, in a barren, scrubby valley with dramatic granite hills behind us. Virtually no planes flew overhead, and only very occasionally did a distant car or freight train create noise. Much of the day there was a great deal of wind, but at twilight and early in the morning the winds calmed down and the quiet revealed itself. Overnight I heard the silence being interrupted only once, when a pack of nearby coyotes howled like ghostly babies.

Early on the second morning, while I was waiting for Diane to set up some recording equipment, I had a chance to contemplate real silence. The ear is exquisitely sensitive. When perceiving the quietest murmur, the tiny bones of the middle ear, which transmit sound from the eardrum to the inner ear, vibrate by less than the diameter of a hydrogen atom. Even in silence, tiny vibrations of molecules move different parts of the auditory apparatus. These constant movements have nothing to do with sound; they stem from random molecular motion. If the human ear were any more sensitive, it would not hear more sounds from outside. Instead, it would just hear the hiss generated by the thermal agitation of the eardrum, the stapes bone of the middle ear, and the hair cells in the cochlea.

On the dunes, I could hear a high-pitched sound. It was barely audible, but I worried that I might be experiencing tinnitus—that is, ringing in the ears, perhaps evidence of hearing damage caused by my excessively loud saxophone playing. Medics define tinnitus as perceiving sound when there is no external source. For 5 to 15 percent of the population tinnitus is constant, and for 1 to 3 percent of people it leads to sleepless nights, impaired performance at tasks, and distress.

Theories of tinnitus abound, but most experts agree that it is caused by some sort of neural reorganization triggered by diminished input from outside sounds. Hair cells within the inner ear turn vibrations into electrical signals, which then travel up the auditory nerve into the brain, but this is not a one-way street; electrical pulses flow in both directions, with the brain sending signals back down to change how the inner ear responds. In a silent place, or when hearing is damaged, auditory neurons in the brain stem increase the amplification of the signals from the auditory nerve to compensate for the lack of external sound. As an unwanted side effect, spontaneous activity in the auditory nerve fibers increases, leading to neural noise, which is perceived as a whistle, hiss, or hum. Maybe what I was hearing on the dunes was the idling noise of my brain while it searched in vain for sounds.

A former colleague of mine, Stuart Bradley from Auckland University, has visited Antarctica, another place devoid of vegetation where silence can be heard. Stuart is a tall New Zealander, sporting a fine mustache like a soccer player from the 1970s. Ironically, what Stuart does in Antarctica is make noise and briefly ruin the pristine natural soundscape. He uses a sodar (a sound radar system) to measure weather conditions, sending up strange chirps that bounce off of turbulent air in the atmosphere before returning to the ground to be measured.

I asked Stuart if he had experienced silence in Antarctica, and he told me about his time in the dry valleys, possibly the most barren places on Earth, which lack snow and ice cover: “Sitting up on the valley wall on a still day, there was no sound I could identify (except heartbeat? breathing?). No life (apart from me). So no leaves either. No running water. No wind noise. I was certainly struck by the primeval ‘feel.’” Stuart commented on how different this was than the sound of a silent laboratory, “I didn’t get the claustrophobic feel one can get in an anechoic chamber ...I suspect this is because, although it was incredibly quiet, it was also a very, very open vista. The valley walls were 1,500–2,000 meters high, and the visibility was amazing!”

Vaults of Silence

My university has one of those anechoic chambers: an acoustically isolated room that provides unchanging, guaranteed silence, uninterrupted by wind, animals, or human noise. It never fails to impress visitors, even though the entrance is utilitarian and uninspiring. Just outside the entrance they see dusty metal walkways, and nearby, builders are often making lots of noise constructing test walls in a neighboring sound laboratory. Guarding the anechoic chamber are heavy, gray, metal doors. You have to go through three doors to reach the chamber, because it is a room within a room. To make the place silent, several sets of heavy walls insulate the innermost room, stopping outside noise from entering. The chamber is mounted on springs to prevent unwanted vibration from getting into the inner sanctum.

Inside, the chamber is the size of a palatial office. First-time visitors are usually circumspect, not least because the wire floor is like a taut trampoline. With the doors closed, they notice vast wedges of gray foam covering every surface, including the floor beneath the wire trampoline. When showing visitors around, I like to say nothing at this point because it is fun to watch the realization sweep across their faces as they adjust to this unbelievably quiet space.

But it is not silent. Your body makes internal noises that the room cannot dampen. Sound recordist Chris Watson described his experience in such a chamber: “There was a hissing in my ears and a low pulsing that I can only guess was the sound of my blood circulating.” The internal noises are not the only oddity. The foam wedges on the floor, ceiling, and walls absorb all speech; there are no acoustic reflections. We are used to hearing sound bouncing off surfaces, which is why a bathroom is lively and reverberant, and a bedroom is muffled and subdued. In the anechoic chamber, speech sounds very muffled, like when your ears need to pop in an airplane.

According to the Guinness Book of Records, the anechoic chamber at Orfield Laboratories in Minneapolis is the quietest place in the world, with a background noise reading of –9.4 decibels. If you chatted with someone, your speech would measure around 60 decibels on a sound-level meter. If you stood quietly on your own in a concert hall, the meter would drop down to a level of about 15 decibels. The threshold of hearing, the quietest sound a young adult can hear, is about 0 decibels. The test room at Orfield Laboratories, like the chamber at Salford University, is far quieter than that.

An anechoic chamber has an impressive silence because it simultaneously presents two unusual sensations: Not only is there no external sound, but the room puts your senses out of kilter. Through their eyes, visitors obviously see a room, but their ears hear nothing that indicates a room. Add the claustrophobic drama of being enclosed behind three heavy doors, and some begin to feel uneasy and ask to leave. Others are struck with fascination at the oddness of the experience. I know of no other architectural acoustic space that regularly has such a strong effect on people. But it is remarkable how quickly the brain gets used to the silence and the contradictory messages from the senses. The magical impact of the first visit to an anechoic chamber is never really experienced again.

Anechoic chambers are rare; a modern concert hall is one of the quietest places most people are commonly able to experience in a city. At the Bridgewater Hall in Manchester, England, tour guides like to recount the story that when the largest peacetime bomb ever detonated in Great Britain exploded in 1996, workers within the auditorium did not hear the bang, because the hall was so well isolated from the outside world. Planted by the Irish Republican Army (IRA) in the city center, the bomb destroyed shops, broke virtually every window within a kilometer radius, and left a 5-meter-wide crater.

It is worth taking the backstage tour of a modern concert hall to see the precision needed to achieve the noise isolation. The tour guides are usually very proud of the fact that the auditorium is built on springs. Like a souped-up car suspension, the springs stop vibration from entering the hall. If ground vibration were to set parts of the auditorium moving, the vibrations of the hall would set air molecules into motion, creating audible noise. Everything connected to the hall that might transmit vibration—electricity cables, pipes, and ventilation ducts—has to be carefully designed with its own little suspension systems. In a good modern hall, the collective noise from audience members breathing and shuffling in their seats is louder than any sounds from outside noises or ventilation systems.

Seeking Quiet, on Earth and Beyond

Outer space is the ultimate silent environment—or, to quote the catchy tag line from the 1979 movie Alien, “in space, no-one can hear you scream.” For an astronaut unfortunate enough to be caught outside the spaceship without a space suit, screaming to occupy the moments before asphyxiation would be pointless, because there are no air molecules to carry the sound waves. When I think of the inside of a real spacecraft, I picture people floating serenely and gracefully in zero gravity. I met NASA astronaut Ron Garan in early 2012, when he had just returned from a six-month mission on board the International Space Station (ISS). He explained to me that the sonic environment in a real spacecraft is a long way from being serene.

Even outside on a spacewalk (his previous mission had included a walk that lasted six and a half hours), there is no silence. Indeed, it would have been worrying if there had been, Garin told me, because it would have meant that the pumps circulating air for him to breathe had stopped working. Spacecraft are full of noisy mechanical devices, such as refrigerators, air-conditioning units, and fans. Inside the ISS it was initially so loud that some feared for the astronauts’ hearing. At its worst, the noise level in sleep stations was about the same as in a very noisy office (65 decibels). Astronauts sometimes had to wear earplugs to deal with the hostile soundscape. The higher levels of carbon dioxide and atmospheric contaminants that exist at zero gravity inside the station might also make the inner ear more susceptible to noise damage.

Although sound levels in the ISS have been reduced enough that they probably no longer pose a risk to hearing, noise can affect health in other ways. For example, someone whose sleep is disturbed by airplane noise is more likely to be tired, irritable, and less effective at work the next day. If we are exposed to high levels of noise, our bodies will produce more stress hormones in the long term that might elevate blood pressure and increase the risk of heart disease. Removing noise is therefore good for us, but is a strong dose of quiet better? Should we be seeking out complete silence?

Going to the countryside and getting some peace and quiet is an obvious response, but rural places are usually far from silent. Within a crowded and intensely farmed country, it is difficult to escape the sounds of agriculture and human activity. Gordon Hempton, a sound recordist and acoustic ecologist from Washington state, has been campaigning for the preservation of natural silence, which “is as necessary and essential as species preservation, habitat restoration, toxic waste cleanup, and carbon dioxide reduction.” He claims that there are very few remaining quiet places in the United States, even though the country has large tracts of empty land. True freedom from artificial sounds is surprisingly hard to achieve because of the web of flight paths that crisscross the sky.

Hempton has named a niche that is free from aircraft noise as “One Square Inch of Silence” and says this “is the quietest place in the United States.” It is located in the Hoh Rainforest, in Olympic National Park, Washington State. But there is not a complete absence of sound. Although the location is free from man-made noises, there are plenty of natural sounds to listen to in this enchanting rainforest. The lush, green canopy of ancient coniferous and deciduous trees with mosses and ferns blanketing the surfaces is home to noisy animals and birds, and the high level of rainfall produces lots of river noises. Imagine if it were truly silent, so the rapid staccato notes of the winter wren were missing and no Douglas squirrel was crying “pillillooeet.” This would be a barren and lifeless place. An essential element of natural tranquility is not silence but what the U.S. National Park Service calls “natural soundscapes.”

A tranquil soundscape has a quality and emotional connection similar to that found in churches: People become very sensitive to the sounds around them, but not in a stressful way. Maybe this sense of spirituality simply reflects the reduced cognitive load on the brain, which experiences less stress when processing the calmer soundscape. To keep our hearing open to possible signs of danger, our brains have to constantly work to suppress unchanging noises, such as the perpetual drone of traffic. This situation is not conducive to finding a relaxed and spiritual sense of well-being.

Ultimate Urban Tranquility

More than half the world’s population now lives in cities. Is it possible to achieve some form of tranquility in an urban soundscape as well? Engineers have been working hard to make cars quieter, but with more traffic on the road, average noise levels in cities have stayed the same. As drivers try to avoid traffic by leaving for work before or after rush hour, their noise pollutes peaceful times of the day; as they try to avoid gridlock, they turn calm back roads into rat runs, ruining quiet places.

Policy makers interested in creating and preserving tranquil urban refuges seek a simple metric that could be measured on a sound-level meter or predicted in a computer model. A scientific report once suggested that areas with sound levels below 55 decibels (a level you might hear from a cheap refrigerator) should be designated quiet areas; another, that artificial sounds should be below 42 decibels (a typical level for a library). By those criteria, there are no tranquil areas in major cities such as London, which is nonsense. Like all world capitals, London is a noisy place, but turn a corner and go down a back street and often you find a quiet square where noise is distant and less intrusive. This just illustrates the problem of trying to reduce human perceptions to simple numbers.

In a city, what matters is relative quiet rather than absolute loudness. As in the countryside, artificial sounds need suppressing, but they do not have to be inaudible. Songbirds, rustling leaves, and moving water need encouraging, because studies have shown that perceived tranquility in towns is greater when natural sounds are louder. Other senses need to be considered: Research suggests that places with more hard landscaping need to be quieter than greener refuges, and certain smells, especially the stench of street urination, do not aid tranquility.

How might you make such an acoustic oasis? Layout is important because a noise source out of sight is usually quieter. The piazza in front of the British Library in London is an interesting example. Facing onto a busy street, it is still possible to find some quiet in this pedestrianized square because a high wall hides the road. Unfortunately, bass sound carries over the wall more easily than higher frequencies, so the rumble of waiting buses is still overpowering from time to time. A higher wall placed closer to the road could solve this problem. In quiet back streets, it is the buildings that often act as barriers to noise.

Hildegard Westerkamp, a world-renowned composer, radio artist, and sound ecologist from Canada explained to me on the London soundwalk that this “quiet stone sound of back streets” is heard only in old cities where roads are narrow and buildings close together. In North America, where most of the streets are wide, it is also hard to escape the hum and whine of ventilation fans. Because lowering sound levels at the source is the most effective strategy, having fewer, slower cars is helpful, as is reducing rolling noise through better asphalt and tire design.

I think scientists are also overlooking an important category of sound that could be good for us. The quiet chatter of people in a café is relaxing and not too alerting. Furthermore, one should have a positive emotional response to being surrounded by other people in a friendly atmosphere. After all, being a social animal has been a vital part of our evolutionary success. Perhaps hearing human activity reduces stress, and is even better than silence.